The present invention relates generally to the fabrication of semiconductor light emitting devices (LED). In particular, the present invention relates to an LED with improved luminance efficiency.
In a conventional white LED package for lighting and display applications, the GaN LED chip is attached to a lead frame and sealed in a resin containing phosphor particles. The phosphor particle absorbs the blue radiation from the chip and emits light in yellow-green and red. The overall light output of the package is perceived as white to human eyes. There are several issues in this structure. For example, a significant portion of the light emitted from the active layer is trapped within the chip and is lost before exiting the chip end. This is due to wave guiding from internal reflection. The trapped light is reabsorbed when reflecting across the active layer. As a result, the luminance efficiency of the LED is impaired. This is generally referred to as light trapping in the current LED. Moreover, it causes uneven conversion of light by the phosphor particle in the package. The light output profile from the package is unsymmetrical and a complex lens is often required to shape the beam profile.
The problem of light trapping can be relaxed by implementing a scattering surface to minimize the wave guiding effect. This can be achieved by roughening the chip surface by using chemical etching from the top surface after the growth of the LED is complete. Surface roughening can be also achieved by depositing the top layer under a less optimal growth condition such as at a lower temperature and a low V/II ratio. Alternatively, scattering surface structures can be formed on the substrate. However, these techniques often require the use of complicated processing procedures and the fabrication cost is high. Layer deposition under less optimal growth conditions adversely impacts the quality of the LED layers and may decrease the device efficiency. Etching from the top surface after growing the LED structure excavates a significant portion of the land from the estate. A significant portion of the LED material grown using the expensive MOCVD process is stripped off and wasted. The active area available for light generation is largely sacrificed thus the power output of the device is low. Since the rough surface is formed after the LED structure is grown, it does not address the problem related to misfit defect and crack formation.
There are other issues with the current LED package. In a typical LED package, the phosphor particles are either suspended in the resin or settled to the bottom of the lead frame. In brief, the phosphor powder and the resin gel are weighed separately and mixed together. A minute amount of the mixture is metered and dropped in the package to seal the LED chip. The package lead frame is transferred to an oven and kept at a fixed temperature and humidity just waiting for the powders to settle. After that, it is brought to a high temperature and the resin cures into a solid. Each step causes process variations leading to a wide spread of the output color of the packaged device. This is generally referred to as color binning on the CIE chromaticity diagram. Special techniques are in order to achieve a uniform light output in a narrow color bin using a different scheme for the phosphor incorporation, but these techniques often produce further complications in the fabrication of the white LED.